Recently, many kinds of solid-state memories which record data based on new principles have been suggested. In particular, as a solid-state magnetic memory, a magnetoresistive random access memory (MRAM) comprising a magnetoresistive tunneling junction (MTJ) element using the magneto resistance (MR) effect is known. Currently, the MRAM stores the information of 0 or 1 depending on the state of magnetization of the storage layer of an MTJ element.
The MTJ element includes a magnetic storage layer (hereinafter, storage layer) and a magnetic reference layer (hereinafter, reference layer). In the storage layer, data is stored and the magnetization is variable. In the reference layer, the magnetization is fixed and does not move. When the direction of magnetization of the storage layer is parallel to the direction of magnetization of the reference layer, the resistance is low (0 state); when anti-parallel, the resistance is high (1 state). This difference in resistance is used to determine the information.
As a method for writing data to the MTJ element, a current magnetic-field-writing system (hereinafter, magnetic-field writing) by the current magnetic field from a bit line is known. In this system, lines are provided near the MTJ element. By the magnetic field produced by the current flowing through the lines, the magnetization of the storage layer of the MTJ element is inverted. When the MTJ element is made small to miniaturize the MRAM, the magnetic field necessary to invert the storage layer of the MTJ element increases. In sum, the magnetic coercive force Hc is increased. Thus, in the magnetic-field-writing MRAM, a large current magnetic field is required in association with the development of the miniaturization, and the write current is large. As a result, it is difficult to realize both the miniaturization and the low current of the memory cell for the purpose of increasing the capacity.
To solve this problem, a writing system (hereinafter, spin transfer torque writing) using spin-momentum transfer (SMT) is suggested.
In a spin transfer torque writing MRAM, the state of magnetization of the storage layer of the MTJ element is changed (inverted) by vertically passing current into the film surface of each film constituting the MTJ element.
In the magnetization inversion by spin transfer torque, the current Ic necessary for magnetization inversion is defined by the current density Jc. Therefore, if the area of the surface of the MTJ element in which current passes is reduced, the injection current Ic for inverting the magnetization is also decreased. In a case where writing is performed with a constant current density, when the MTJ element is small, the current Ic is also small. Thus, the spin transfer torque writing system is, in principle, scalable. With the spin transfer torque writing system, an MRAM with a large capacity has become possible.
The above-described MTJ element (magnetic element) is covered by a protective insulating film. However, generally, the coefficient of thermal expansion of the material contained in the MTJ element (magnetic element) is greater than the coefficient of thermal expansion of the material contained in the protective insulating film. Therefore, a thermal stress may be applied between the magnetic element and the protective insulating film, and have a negative influence on the characteristic and reliability of the magnetic memory device.
In view of the above factors, an MRAM comprising an MTJ element in which the stress between an MTJ element (magnetic element) and a protective insulating film is reduced is desired.